The advantages of the AlGaAs/GaAs heterojunction bipolar transistor (HBT) in high speed applications result both by virtue of its structure and from the superior carrier transport properties of GaAs. In comparison with homojunction bipolar transistors, HBTs simultaneously allow application of heavy base doping to reduce the base resistance, and low emitter doping to reduce the base-emitter capacitance while still maintaining excellent current gain. Together with the availability of semi-insulating substrates and higher electron mobility, HBTs in GaAs have achieved much higher values of f.sub.T and f.sub.max than the best Si bipolar transistors. In comparison with GaAs field-effect transistors (FETs), the superior properties of HBTs include; high transconductance due to the exponential current increase as a function of input voltage, and uniform threshold voltage because it is controlled by the built-in potential of the base-emitter junction. The threshold voltage is thus much more uniform than for a FET. Furthermore, HBTs are vertical structures in which the carrier transit times are established by epitaxial layer design, rather than critical lithographic dimensions. High performance HBTs can thus be fabricated without the expense of submicrometer lithography.